Nebulizers generate an aerosol output, and are used for medication delivery through the respiratory channel. The patient receives a specific amount of medication in the form of small droplets (aerosol) that are formed by forcing the medication through a mesh in the form of a thin metal plate with tiny holes.
The volume of medication to be nebulized (typically 0.2 to 2 ml) is dosed into the device, and the device generates the aerosol by means of well known methods such as a vibrating mesh, vibrating horn, or a vibrating flat plate in a resonant cavity. The required ultrasonic vibration is generated by an actuator, typically a piezoelectric crystal. The amount of medication that reaches the patient during the treatment is equal to the supplied medication dose minus the aerosol deposited in the device and residues of medication that remain in the device after the treatment is finished.
For a medication therapy, it is sometimes required that not only the dose is precisely defined, but also the rate at which the medication is delivered, namely the aerosol output rate. The nebulizer generally controls the aerosol output rate by means of the electrical power and driving frequency applied to the piezoelectric drive system.
The aerosol output rate cannot be exactly predicted based on the applied electrical power. Aerosol generating systems may have different efficiencies (amount of aerosol generated per unit electrical power), for example due to device and mesh tolerances, temperature, and cleanliness of the mesh.
A system that estimates the aerosol output rate by measuring the density of the aerosol beam and the air flow rate may be used in a feedback control loop to adjust the electrical power. The aerosol density can be measured by means of an optical beam perpendicular to the aerosol beam. The optical beam can be generated by a light emitting diode (LED). The beam shape of the light from a LED is divergent, and the optical beam may be collimated to a parallel or nearly parallel beam using one or more lenses. The beam may be further shaped using for example a circular or rectangular diaphragm.
The optical beam crosses the aerosol beam, and falls on an optical sensor (optionally through a diaphragm and optionally focused using one or more lenses). The optical system can be calibrated by measuring the sensor signal at a time that no aerosol is present with the LED off (“dark signal”) and with the LED on (“light signal”). If the aerosol beam is present, the rays of the optical beam are scattered by the droplets, thus decreasing the light that falls on the optical sensor, and hence decreasing the measured output signal at the optical sensor. The decrease of light on the sensor caused by droplets in the light path is called obscuration. The obscuration can be quantitatively expressed by the parameter (“light signal”-“measured signal”)/(“light signal”-“dark signal”).
The obscuration is a function of the droplet density in the aerosol beam and the length over which the light travels through the aerosol beam. If the velocity of the aerosol beam is known, e.g. through a separate air flow rate measurement (using a differential pressure sensor or a flow sensor), then the aerosol output rate can be computed from the aerosol density and the volume of the aerosol beam that passes the optical beam per unit of time. The volume can be calculated from the product of the cross-sectional area of the aerosol beam and the velocity of the aerosol beam.
The method described above of estimating the aerosol output rate from optical beam obscuration and air flow has the disadvantage that two detection systems are required: the optical system and the air flow measurement system.
The optical system may be located either at a distance from the mesh such that the air flow and droplet velocity are the same, or closer to the surface of the mesh where the droplets are ejected. In this case, the droplet velocity is not strongly related to the air flow but dependent on the nebulizing parameters such as power and frequency of the driver electronics). Both regimes have their own disadvantages.
For an optical system at a distance from the mesh, the aerosol droplets have the same velocity as the surrounding air flow, but aerosol droplets may be deposited on the optical system (e.g. on the lenses) such that the detected signal on the optical sensor decreases. This reduces the reliability of the density measurement, because it is not possible to distinguish if the signal decrease is caused by the aerosol density or the contamination of the optics. This disadvantage may be overcome partially by calibration at regular intervals.
Depending on the mechanical design of the nebulizer, it may be necessary to locate the optical system in the mouthpiece of the nebulizer. Since the mouthpiece is usually a detachable and replaceable part of a nebulizer, the location of the optical system may create design difficulties and increasing costs.
For an optical system close to the mesh, it is possible to construct the optical system in such a way that contamination is to a large extent avoided, but the disadvantage is that the average velocity of the droplets close to the mesh is defined by the aerosol generation system instead of the air flow. Moreover, the average droplet velocity generally increases with higher aerosol output rate. As a result, the density of the aerosol beam (as detected by the optical system) increases only marginally with the aerosol output rate, and cannot be used as reliable estimate of the aerosol output rate.
US 2006/0087651 discloses a system in which the aerosol velocity is obtained by particle image velocimetry, by which particles are mapped through successive images to derive flow vectors for the individual particles. This is a computationally intensive and hardware intensive process.